Tsunamis on Lake Geneva

Lake monsters

A millennium and a half ago, Geneva was destroyed by a giant wave. Recent research suggests it could happen again

IN 563AD a tsunami devastated Geneva. Two accounts of the disaster, one by Gregory of Tours and the other by Marius of Avenches, have survived. What caused the wave, and the extent of the damage that resulted, have been matters of conjecture for centuries. But over the past decade several groups of scientists have pieced together the sequence of events and one of those groups, led by Katrina Kremer of the University of Geneva, has now created a computer model of what happened. Unfortunately for the 1m people who live around the lake’s shore, the conclusion of this research is that something similar could easily happen again.

The tsunami of 563 started at the opposite end of the lake from Geneva, at the point where it is fed by glacial meltwater carried into it by the Rhône. Both accounts say the wave began with a massive rockfall on what was then called Mount Tauredunum (this has led to the tsunami becoming known as the Tauredunum event). Tauredunum is thought to be a mountain now called the Grammont, which is located near the river mouth.

In the past, one favoured theory was that this rockfall created a natural dam across the Rhône, which held the waters back until it could no longer sustain the pressure. When the dam burst, the resulting wave swept the length of the lake. A one-off event, in other words. But a paper just published in Nature Geoscience by Dr Kremer and her colleagues offers a different and more worrying explanation.

Canyon diabolo

Dr Kremer thinks that the rocks crashed down onto soft sediments which had accumulated at the river mouth because of the slowing of the river’s flow when it enters the lake. These sediments form an underwater delta that has several canyon-like channels. When the falling rocks hit the delta they destabilised the sediments and caused the canyons to collapse. It was this collapse that created the tsunami.

It is a plausible theory. What suggests it is true is that the sediment from such a collapse would have been propelled towards the lake’s centre, forming a large tongue of material on the lake bed. And, using sediment cores and an instrument called a pinger, which analyses the reflections of sound waves that can penetrate the material of the lake bed, Dr Kremer thinks she has found this tongue.

Her discovery is a bed of what is known geologically as turbidite. This is sediment that, because it is laid down by rapid water movements, is not sorted by grain size. The turbidite Dr Kremer found is a mixture of sand and silt roughly 10km (six miles) long and 5km wide. On average, it is five metres deep, and it seems to have formed in a single event. By carbon-dating leaves and other organic material trapped within it, she has shown that it is about the same age as the Tauredunum event.

The discovery of this turbidite means it is possible to work out what happened in 563. The result is not pretty. The team’s computer modeller, Guy Simpson, has reconstructed the tsunami that would have been generated by the amount of material in the tongue. Within 15 minutes of the collapse, a wave 13 metres high would have reached Lausanne, a city on the northern shore of the lake. But Lausanne is built on steep slopes, so most of it would have been spared. The damage would have been much greater when, 55 minutes after that, an eight-metre wave reached Geneva, at the other end of the lake. Geneva is a lower-lying city than Lausanne and, to make matters worse, the lake narrows here, funnelling water to the point where the Rhône becomes a recognisable river again. It would similarly have funnelled the wave.

Using information revealed by archaeology about the layout of sixth-century Geneva and the lake’s level at the time, the team were able to work out how the city would have stood up to the onslaught. The wave, they believe, would have passed over the city walls, and wiped out watermills and a bridge across the Rhône, just as the two accounts say it did. En route, as Marius wrote, it would have destroyed many lakeshore villages, “with humans and cattle and even churches”.

The crucial element in this explanation is the accumulation of sediment in the underwater delta. That is a continuous process. Once enough sediment is there, it just needs a trigger to set it going. That could be a rockfall, an earthquake (though this part of the world is not particularly seismically active) or even a violent storm. Moreover, Dr Kremer’s pinger shows evidence of four layers deeper in the lake bed which also look like turbidite. The formation of these might or might not have triggered tsunamis. But they are a worrying sign.

Though the basin in which Lake Geneva sits is ancient, the modern lake is a product of the end of the last Ice Age. Exactly when it formed is unclear. The whole area was still buried under ice 19,000 years ago. By 13,000 years ago the glaciers had retreated at least as far east as Lausanne. But the age of the current delta is still unknown. That five layers of turbidite may have formed in this time gives a rough sense of how frequently tsunamis might happen. The details will remain obscure, though, until the older beds are examined closely, and core samples taken from them.

Several things thus need to be done. The most urgent is an assessment of the state of the underwater delta. This would try to work out the risk of it collapsing again in the near future, and also ask how much dredging would be needed to reduce that risk. Second, the older turbidite beds need to be examined properly, to see how frequently tsunamis actually occur. Third, it might behove the authorities in Geneva and elsewhere around the lake to assess which areas would be inundated if there were another tsunami, and how they might react if there was one—for if a tsunami did happen, the Genevese would have just over an hour to evacuate their city before its centre was obliterated.

Dr Kremer’s work also raises the question of whether other lakes are at risk of generating tsunamis. Some might be. In 1806, for example, a landslide into Lake Lauerz, further east in Switzerland, triggered a tsunami 20 metres high. From Loch Ness in Scotland to Tele in Congo, lakes breed legends of monsters lurking beneath the surface. Perhaps, in some cases, the events behind these legends are real.

I am certain the Swiss will work out a game plan for preventive action, legislate the funds, and get it done. They are rational. I would shudder if Geneva were in the US. One part of the electorate would call it a liberal conspiracy, another overlapping part would deny the needed funding as "no new taxes", and another overlapping part would say the resulting disaster was "the will of God" when the predictable event occurred. Another part would do a cost/benefit analysis, conclude some lives lost would be a sad but acceptable loss, and conclude a net benefit from the resulting rebuilding. Such is our part of the world today.

This is a fascinating bit of geological, hydrological, and historical research. Thanks for reporting on some science that is comprehensible to a layman, and is not about medicine, charismatic megafauna, or climate change (at least not directly).

This discovery obviously argues for more research and more funding, as almost every scientific discovery does. What's not mentioned here, however, is how conditions may have changed in recent centuries to make a similar event more or less likely in different lakes that are prone to underwater sediment slides. Specifically, two things that people do to watersheds have drastically altered the amount of sediment entering natural lakes. One is to build dams upstream that collect sediment and prevent it from flowing as it naturally would. The Glen Canyon dam is the most famous example of a dam drastically altering the downstream environment by trapping sediment. The other thing is to increase sedimentation by denuding the watershed through logging and farming practices. So, even if the sediment record shows a fairly regular regular repetition rate for these slides, all bets are off once humans got into the area and started mucking (literally) with the sedimentation rate.

Of course waves can take curves; I live in a coast facing south, and I mostly surf waves coming from North-West.
(tsunami waves are different because they mostly are underwater, while ocean waves are surface caused by the wind, and have a higher wavelength but their dynamics are quite similar; they bend, refract, are amplified, etc)
Anyway, in this case, the wave would not be underwater, since it would be caused by a landfall .

A couple of figures would have been worth all 1100+ words of this article. I've now read it twice and, despite some experience reading sedimentology papers, I can't make much sense of it. For example: could a drowned canyon really collapse fast enough along sufficient length to provide a tsunami-level wave? If the impulse were due to the collapse of a canyon, why would the unsorted sediment form a front 5 km wide, rather than just a bit wider than the canyon? How does this differ from the results of, say, a partial ice-dam collapse? I suppose it will send me to the library to read the paper one day (if the local university can still afford the Nature specialty journals). That's good thing, but I wish TE, having aroused my interest, could have saved me the trip.

Geneva Lake is in a shape of banana with city of Geneva at one end and Rhone mouth at the other there is no straight line between them. Tsunami travels in a straight line. Given the location of the wave’s origin the waves would have be drastically minimized at Geneva.

Hi,
Tsunamis on Lake Geneva. Lake monsters. “What faults they commit, the earth covereth”. Whet the earth has Parkinson's things shack. I was aware of this and a few years ago I was concerned at the environmental effect of CERN. First of all the high synthetic gravitational field at a time when the gravitational pull of the Universe reaches a high within a 5000 year cycle I though could course damage. In addition I was concerned that the high magnetic field in and around CERN would course a higher incidence of Hodgkin's lymphoma. I informed the then President of Switzerland who talked to the people at CERN who thought I was mad. I hope I am mad.

Sir,
The ice-age in which we are currently living, called the Pliocene-Quaternary, started about 2.5 million years ago. Within the same ice-age, there have been glacial and interglacial periods, lasting between 40k and 100k years. We are currently in an interglacial period, but still within the same ice-age as there is still ice in the Polar Regions, Greenland and elsewhere. Previous ice-ages have been recorded in the geology of the earth; Huronian 2.4 – 2.1 billion years ago, Cryogenian 850 – 630 million years ago, Andean-Saharan 460 – 420 million years ago, and Karoo 360 – 260 million years ago. By the measure of past ice-ages, it would appear that ours is just getting started rather than have ended as you state.